This invention relates to a bonded ceramic metal composite substrate having a copper sheet directly bonded to a ceramic substrate, a circuit board constructed of such a bonded composite, and to a method for their production. In recent years attempts have been made to develop ceramic-metal composite substrates for use in electronic devices. For example, a capacitor element bonded to a ceramic aluminum nitride substrate is disclosed in Scace et al., U.S. Pat. No. 3,716,759. Direct bonding of metal sheets to ceramic substrates is also described in Hill, British Pat. No. GB 2,099,742 and Jochym, U.S. Pat. No. 4,409,278, whereby channels are provided in the metal sheet or ceramic substrate in order to avoid blistering by permitting gas escape during the bonding. Some of these attempts have involved disposing a metal sheet such as copper on a ceramic substrate and directly bonding the copper sheet to the substrate by heating to a temperature below the melting point of copper (1083.degree. C.) and above the eutectic point of copper and oxygen (1065.degree. C.).
This kind of composite has several advantages. It provides a strong adhesion between the metal sheet and the ceramic substrate. It also provides a simpler structure which can be easily utilized to obtain a smaller circuit board. And it provides a potentially simpler method for producing a circuit board.
However in attempting to use ceramic copper substrates prepared in the foregoing manner in circuit boards for transistor modules, the present inventors found that some of them failed to work correctly during their operation. Thus, there is a need for bonded ceramic metal substrates which will produce more reliable electronic devices.